Volcanoes
 What a Volcano Is. Another terrible manifestation of the interior forces of the earth are volcanic eruptions. In a number of places within the crust of the earth there are concentrations of molten masses (magma) containing numerous gases.
Fig. 22. A section of a volcano:
1 —The concentration of magma; 2—layers of rocks; 3—the opening through which the molten masses are erupted; 4 — the old cone, blown away in former eruptions; 5—the new cone and its crater; 6—layers of hardened lava, or coulees; / — cracks containing magma that has not reached the surface of the earth.
In places where the earth's crust is weak and cracked, the magma, under the pressure of the gases, bursts through the surface, thus forming a volcano. A volcano is a conical mountain with a hollow top called a crater (from the Greek «krater»=mixing-vessel). The eruption occurs through the vent in the crater (Fig. 22).
First comes an underground rumble. As the ejection of gases and steam increases, they rise in a gigantic column which widens, forming a dark cloud. Volcanic ash is forced out of the crater together with the gases and steam. It covers the surrounding area with a layer so thick that it forces in the roofs of houses. It becomes pitch dark. Down come torrents of rain, accompanied by flashes of lightning and peals of thunder. The rain turns the ash into mud, swift streams of which rush downward, flooding the neighboring villages and even towns. Fragments of heated rock are thrown out by the volcano. The crater is finally filled to the brim with molten lava, and the fiery liquid (1,000° to 1,300°C) streams slowly down the slopes, burning everything in its way. The surface of the stream soon cools. The volcanic cone usually consists of alternate layers of solid lava and ash.
 Volcanoes of Europe. There are active volcanoes today near the coasts of the Mediterranean, where the earth's crust is cracked in many places. On the western coast of the Apennine Peninsula rises Vesuvius.
Fig. 23. An eruption of Vesuvius. Fragments of flying rocks may be seen. There are coulees all around.
In a violent eruption in the first century this volcano
destroyed a number of towns and villages of ancient Rome, covering them with hot volcanic ash and flooding them with torrents of mud Vesuvius is still active today, ejecting steam and gases, sometimes ash, fragments of rock and even streams of molten lava (see Figs. 23, 24, 25). On the island of Sicily there is another active volcano— Etna.
Another region of volcanic activity is Iceland, where there are
many active and extinct volcanoes, and where solid lava covers practically all the surface. The
most notable volcano here is H e k 1 a. Vast glaciers cover the highest regions and volcanoes of Iceland. During eruptions the ice melts, and swift streams of water rush downward, causing great destruction.
 Geysers. There are many hot springs in Iceland and other volcanic regions. A hot spring, the water of which at regular intervals is shot upwards in fountain-like columns, is called a geyser.
Fig. 24. Vesuvius and neighbourhood.
Along the roads surrounding the volcano are villages, frequent victims of its eruptions. In toe south-east are the ruins of the ancient Roman city of Pompeii. A highway and a funicular railway (a funicular railway is a cable railway for taking people up and down mountains) lead to the top of the volcano. Use the scale to determine the distance from the crater to: a) the outskirts of Naples;
The mouth of the geyser contains water which becomes heated from underneath and begins to boil. A hollow gurgle is heard.Clouds of steam burst forth from the opening, and a fountain of boiling water shoots up with a roar, rising to a height of dozens of metres. The geyser then calms down, waiting for a new eruption.
In Iceland, natural hot springs are used to heat town houses, as well as hotbeds and greenhouses, where fresh vegetables ripen the year round. |
